Craig Tim, Battista Jerry, Van Dyk Jake
London Regional Cancer Centre, Department of Medical Biophysics, University of Western Ontario, London, Ontario N6A 4L6, Canada.
Med Phys. 2003 Aug;30(8):2001-11. doi: 10.1118/1.1589492.
Convolution methods have been used to model the effect of geometric uncertainties on dose delivery in radiation therapy. Convolution assumes shift invariance of the dose distribution. Internal inhomogeneities and surface curvature lead to violations of this assumption. The magnitude of the error resulting from violation of shift invariance is not well documented. This issue is addressed by comparing dose distributions calculated using the Convolution method with dose distributions obtained by Direct Simulation. A comparison of conventional Static dose distributions was also made with Direct Simulation. This analysis was performed for phantom geometries and several clinical tumor sites. A modification to the Convolution method to correct for some of the inherent errors is proposed and tested using example phantoms and patients. We refer to this modified method as the Corrected Convolution. The average maximum dose error in the calculated volume (averaged over different beam arrangements in the various phantom examples) was 21% with the Static dose calculation, 9% with Convolution, and reduced to 5% with the Corrected Convolution. The average maximum dose error in the calculated volume (averaged over four clinical examples) was 9% for the Static method, 13% for Convolution, and 3% for Corrected Convolution. While Convolution can provide a superior estimate of the dose delivered when geometric uncertainties are present, the violation of shift invariance can result in substantial errors near the surface of the patient. The proposed Corrected Convolution modification reduces errors near the surface to 3% or less.
卷积方法已被用于模拟几何不确定性对放射治疗中剂量传递的影响。卷积假设剂量分布具有平移不变性。内部不均匀性和表面曲率会导致这一假设不成立。因违反平移不变性而产生的误差大小尚无充分记录。通过比较使用卷积方法计算的剂量分布与直接模拟获得的剂量分布来解决这个问题。还将传统静态剂量分布与直接模拟进行了比较。针对体模几何形状和几个临床肿瘤部位进行了这种分析。提出了一种对卷积方法的修正，以校正一些固有误差，并使用示例体模和患者进行了测试。我们将这种修正后的方法称为校正卷积。在计算体积中（在各种体模示例中对不同射束排列进行平均），静态剂量计算的平均最大剂量误差为21%，卷积为9%，校正卷积降至5%。在计算体积中（在四个临床示例中进行平均），静态方法的平均最大剂量误差为9%，卷积为13%，校正卷积为3%。虽然当存在几何不确定性时，卷积可以对传递的剂量提供更好的估计，但违反平移不变性会在患者体表附近导致显著误差。所提出的校正卷积修正将体表附近的误差降低到3%或更低。
